Conventionally, leather-like sheets have been used in various fields, and above all, so-called grained artificial leathers having a elastic polymer layer on the surfaces have been contrived with regard to the expression of their appearances. As a method for the appearance expression, there is employed a method of applying a colored pattern to a sheet surface by known printing techniques such as gravure printing, roller printing, etc., a method of applying a colored pattern by transfer or a method of applying a colored pattern by textile printing.
These patterns are expressed on sheet surfaces by means of patterns of a gravure roll, an engraved roll, a printing screen, etc., and for changing color tones and designed patterns, it is required to repeat the engraving of a pattern on the roll and, further, color matching. In these conventional methods, further, the expression scope and color tone of patterns have limitations such as coloring of one color at one step, etc., and the application of a number of fine patterns to a sheet surface have necessitated a large cost and a long time.
In the field of printing, the technique of multi-color printing of an information pattern drawn on a computer screen for a short period of time with an inkjet printer has come to be widely used in recent years. This inkjet printing method has a characteristic feature in that various patterns can be sharply printed in various colors for a short period of time, differing from the above gravure printing, roller printing, transfer or textile printing according to conventional methods. However, printing on a leather-like sheet is not easy unlike printing on paper, and the present situation is that various attempts are being made.
For example, Patent document 1 describes a leather product having an image drawn by forming an aqueous undercoating agent layer on the surface of leather such as natural leather, providing thereon a finely porous ink receptor layer containing an alumina hydrate and inkjet-printing the image on the finely porous ink receptor layer. However, pores in this leather product are pores of an nm order relying on the alumina hydrate. In this leather product, the ink receiving capacity (dyeing property) of the finely porous ink receptor layer is not sufficient. Even if this method is applied to an artificial leather surface, a sharp pattern is hard to form, and the color tone of the pattern is liable to lack depth. In this method, further, alumina hydrate is liable to frictionally fall off, and in view of abrasion resistance, the above leather product has no practical utility that can be applied to artificial leather that is processed into sport shoes, and the like.
Further, Patent document 2 proposes a process for the production of a leather-like sheet, which comprises drawing an image on an easily dyeable layer on a fibrous substrate with a dye ink according to an inkjet printing method and providing a transparent or semi-transparent protective layer on the drawn image. However, this leather-like sheet uses a dye ink and hence has the following defect. When it is processed into shoes, athletic balls or bags which are in particular used outdoors, and when they are actually used, they are not sufficient in light-resistant fastness, and a fine and sharp image obtained by the inkjet printing method cannot be maintained for a long time.
Due to the use of a dye, further, it has a defect that the resistance to color migration is not sufficient. That is, when the surface of a shoe, ball, bag, glove, or the like, which are processed from the leather-like sheet, comes in contact with the surface of a counterpart, the dye migrates to the surface and stains the counterpart that has contacted.
For overcoming the above problem, it is thinkable to use not a dye ink but a pigment ink as a colorant. That is, when a pigment ink was applied to the inkjet printing method, an image can be maintained for a long time even if it is used outdoors. When a pigment ink is used, however, there are defects that the abrasion resistance, anti-peeling property and flexibility of a surface are degraded. That is, when a pattern is formed on a leather surface with a pigment ink, the defect is that the adhesion strength between a layer on which the pattern is formed and a layer adjacent thereto is greatly decreased due to pigment particles that hamper the adhesion. When the leather-like sheet is processed and shaped into shoes, bags, etc., there is caused a problem that a flexed portion undergoes a peeling off or that an image on a flexed portion falls off.
As described above, leather-like sheets having fine images on their surfaces still have room for improvements with regard to abrasion resistance, light-resistant fastness, resistance to color migration, abrasion resistance, anti-peeling property, flexibility, and the like.
(Patent document 1) JP-A 9-59700
(Patent document 2) JP-A 11-158782